1. Field of the Invention
This invention relates to autofocusing cameras having a distance measuring portion which operates with an optical system outside the photo-taking lens. More particularly, the invention relates to a device for permitting selective control of automatic and visual focusing of such cameras dependent upon the absence and presence of an attachment lens at the front of the photo-taking lens.
2. Description of the Prior Art
Many methods for controlling automatic focusing of a camera have been proposed, of which three examples are typical. The first example has a focus adjustment which is effected by detecting the sharpness of an image of an object to be photographed. The second example employs a signal (an ultrasonic wave, for example) which is projected to the object and the reflected signal from the object is received in a time interval based on which the camera-to-object distance is determined to effect automatic adjustment of the fucusing. Finally, another such example makes use of a distance meter whose principle is similar to that employed in trigonometrical surveying. To realize these methods in a camera, a portion of light passing through the photo-taking lens may be utilized in measuring the camera-to-object distance to provide a so-called "TTL distance measuring type automatic focusing adjusting device" (TTL mode). Alternatively, without use of the photo-taking lens, distance information may be obtained through an optical system provided separately from the photo-taking lens, the latter being a so-called "outside-the-lens distance measuring type automatic focus adjusting device" (OTL mode).
For the purpose of broadening photographic capabilities, an optical accessory such as a closeup lens or wide angle lens for attachment is often used on the front end of a mechanical mounting of the photo-taking lens. In the case of a camera whose distance measuring portion operates in the TTL mode, since the degree of focus of an image for distance measurement is made equivalent to that of focus thereof at the film plane independently of whether or not the attachment lens is used, the accuracy and reliability of the autofocusing device are not reduced to any extent by the use of the attachment lens. In the case of the camera operating in the outside-the-lens distance measuring mode, however, since the operating mechanism for the distance measuring optical system is mechanically linked to the autofocusing mechanism in a predetermined relation, the attachment lens contributes to the lack of sharpness of the image at the film plane. It will be evident that the outside-the-lens distance measuring autofocusing camera must be provided with a device for permitting visual focusing while the autofocusing device is excluded when the attachment lens is used.